GRE MTU size and others

From: Al Gesse (groupstudy@mail.ru)
Date: Sun Jul 04 2004 - 15:59:24 GMT-3

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Hello,

Confirm or correct me if I'm wrong

Implementation of IP/GRE tunneling on Ethernet:

IP MTU = 1500
- means | IP(20) | TCP(20) | DATA(1460) |
  so in the wire we'll have 1518 (Ethernet)
  So IP MTU=1500 means "after adding IP header".

Interface MTU = 1500
- means IP packet's size before wrapping Ethernet header
  around IP packet, so we'll have 1518 in the wire.
  So Interface MTU=1500 means "before adding Ethernet header".
  

GRE MTU = 1476
- means | IP(20)original | TCP(20) | DATA(1436) |
  When we add GRE header, it consists actually
  of 20 bytes' new IP header and 4 bytes' proper GRE header.
  So we'll have 1500 before wire and 1518 in the wire.
  And GRE MTU=1476 means "before adding GRE/newIP header".

All the fragmenting stuff is beyond the point till now.-)

Regards,
Al

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